"It’s way too early to tell," James Beckerman, a cardiologist with the Providence Heart and Vascular Institute in Portland, Oregon, told NBC News. "It’s nice to be optimistic, but it's also good to be realistic about what this technology can provide."

Sound body

Before talking to NBC News, Beckerman had just run a leg of the 199-mile long Hood Coast Relay in Portland, where he met a man who was monitoring his diabetic daughter's glucose levels from his smartphone.

The idea of the "quantified self" has real promise for people with chronic diseases, he said, but he wasn't so convinced that his own GPS running watch was really helping him become more fit.

"These trackers do tend to push people in the right direction," he said. The problem is people often abandon, lose or break their fitness trackers pretty regularly. In fact, around one-third of wearable owners abandon them in within six months, according to a report from market research firm Endeavor Partners. Fitness fads, of course, are nothing new.

"When you think of diet books, there are hundreds of them on the shelves each year — there is a reason for that," Beckerman said. People like hopeful new messages and ideas, he said, and it's often those who are in the worst health who are let down by them. Those who succeed and evangelize are often the ones who are already relatively fit, he said.

"I do think that many of the people who have embraced tracking are people who are making those healthy choices anyway, and they are interested in data or like reinforcement for what they already doing."

Sound mind

The picture gets murkier when you start talking about the brain. Lumosity claims to "train memory and attention" with mental exercises that look like simple Web, iOS and Android video games. With 60 million users, it's the big dog on the scene, but there are plenty of others out there like CogniFit and Elevate.

"My general view — and what the literature says — is that these services can help you improve any given task, but the big issue is, 'Do those skills transfer to other things?'" Elliot Berkman, a psychology professor at the University of Oregon, told NBC News.

With weight loss apps, it's easy. Either a fitness tracker did or did not help you lose weight. When it comes to the brain, the results are more ambiguous. Berkman's research indicates that by practicing Lumosity's games, people might just be getting better at those games.

That is fantastic if your goal is become the best in the world at memory games, but not so great if you are trying to improve your memory overall. The plus side?

"It can’t hurt," he said of playing these games, especially if the alternative is "screwing around on the Internet."

Making sense of the noise

One big problem with the "quantified self" movement is the quantifying part. Getting detailed stats on your sleep, sex life and eating habits seems cool, but how do you make sense of all of that data?

The average person is not going to spend time creating data visualizations with the information gleaned from their FitBit. Even experts have trouble dealing with the data, especially when it's from hundreds of different fitness trackers with a unique definition of what constitutes a "step."

"It would definitely be easier if everyone used a Shine [which tracks health and sleep] and an iPhone," John Wilbanks, chief commons officer of Sage Bionetworks, a non-profit that analyzes biological data, told NBC News.

"I'm not saying it would be better — I think it's good to have competition and diversity," he said. "But it's always easier if you don’t have to normalize data from different types of devices."

The potential of wearables

While the marketing might have jumped ahead of the research, there is still hope that tracking could one day make a big impact on people's well-being. Wristbands might be easily abandoned, but most people won't leave the house without their smartphones.

"The phone market just might eat the wearables market," Wilbanks, who has personally lost five wearable devices, said.

Once sensors like the heart-rate monitor in the Samsung Galaxy S5 become standard in all smartphones, we could see more consistent, quality data and more useful services built around that data. Wilbanks gave the example of someone with depression getting help when their phone detects that they have not left the house or talked to anybody in a long time, signs that would otherwise have to be self-reported.

Overall, the idea of the quantified self has "become a Rorschach test for whether you are a utopian or dystopian" about technology, Wilbanks said.

Utopians imagine a world where everyone is engaged in connected studies, quality data comes pouring in and people are nudged towards healthier behaviors. Cynics see wasted money, privacy concerns and health insurance companies colluding with tech companies, Wilbanks said.

"In that happy version, nobody is getting punished on their insurance premiums because they are eating cake. The reality, I think, will be somewhere in-between."